1. The Field of the Invention
The present invention relates to crystalline, substantially single-phase, ceramic compositions incorporating bioactive ionic species that provide biocidal or antimicrobial properties, methods of synthesizing such ceramic compositions and methods for manufacturing microbe-destroying articles using the ceramic compositions and utilizing them.
2. The Relevant Technology
The health and environmental hazards of bacterial contamination from microbes such as Escherichia coli and Salmonella commonly found in food and water, Staphylococcus Aureus present in uncooked or undercooked meat; Cryptosporidium parasites found in water, and other such unicellular organisms, are no less than they have ever been in the past. In fact, with increasing human population, growing pollution and the potential threats of bio-terror, microbial problems have assumed greater dimensions in the present day and age.
For centuries, metals such as silver (Ag), copper (Cu), zinc (Zn), tin (Sn) and cobalt (Co) have been known to be benign antimicrobial agents and have been used for various basic microbe-control applications. Most of these applications utilized the antimicrobial metal in its unalloyed or alloyed form. However, in recent times, silver and copper, in particular, have been used extensively in various other forms with other substances for disinfecting (antibacterial, antifungal and antialgal) applications. About 20 years ago, silver began being used with other materials for antimicrobial coatings, components and devices.
Different amounts of bioactive species have been incorporated into various organic, inorganic, composite and porous substrates to facilitate antimicrobial activity or disinfecting properties. Typical conventional uses of silver are based on physical admixing of silver or its compounds (e.g., silver iodide, nitrate, oxide, sulfadiazine) with a carrier for use in topical medications, dentistry and water treatment, or, depositing the mixture on a surface (e.g., colloidal coating, paste, or a glaze) on, for example, textiles, plastics, kitchen counters or tiles for floors and walls in restrooms. However, many of the prior art silver-based compounds contain higher-than-needed levels of bioactive or antimicrobial dopants (Ag, Cu, Zn, etc.) and yet are not capable of sustained, strong antimicrobial activity over a period of time.
In particular, where the antimicrobial species were physically bonded or admixed with the base material or coated onto a substrate, the antimicrobial activity is likely to degrade rapidly resulting from loss of the antimicrobial (Ag, Cu, etc.) species due to dissolution or degradation phenomena. Compared to the relatively unstable organic and composite biocides, inorganic biocides offer the advantages of intrinsically higher environmental stability, safety (non-toxic) and controlled and prolonged antimicrobial activity.
State-of-the-art inorganic antimicrobials such as AgION™ and Zeomic comprise silver (Ag) or copper (Cu) based zeolites (alumino-silicate based minerals), wherein the silver or copper ions are put in place of metal ions in an open, skeletal network structure. However, in this often porous and open structure, both the host metal ions such as sodium (Na+), potassium (K+) and magnesium (Mg2+) and the dopant ions such as Ag+ or Cu+ are very loosely held making them vulnerable to rapid, uncontrolled ion-exchange and acid leaching. Additionally, silver ions in such zeolites can be easily reduced to metallic silver which could tend to cause coloring of the antimicrobial material and, in turn, the host object.
Alternative inorganic antimicrobial approaches include antimicrobial compositions based on hydroxyapatite, zirconium/titanium/tin phosphate (such as Alphasan™) or silicon dioxide or titanium oxide or zinc oxide (Microfree™) crystalline chemistry. Several variations of the phosphate based inorganic antimicrobial compositions exist, among which the most exemplary are embodied in U.S. Pat. Nos. 5,296,238, 5,441,717 and 5,698,229. For instance, in U.S. Pat. No. 5,296,238, microbicides cover a family of phosphates represented by the general formula:Ma1AbMc2(PO4)d.nH2Owherein M1 is silver, A represents at least one ion selected from the group consisting of hydrogen ion, alkali metal ions, and ammonium ion, M2 is zirconium or titanium, n represents a number which satisfies 0<n<6, a and b each represents a positive number and satisfies the equation la+mb=1, where l is valence of M1 and m is valence of A, and c is 2 and d is 3.
While these prior-art microbicide (U.S. Pat. No. 5,296,238) and antimicrobial (U.S. Pat. No. 5,441,717) compositions represent some of the more physically and chemically stable inorganic materials with potentially pronounced and prolonged antimicrobial activity to date, there are shortcomings associated with the intrinsic stability of the above phosphate compositions. The stability issues arise from the presence of monovalent alkali ions present at the A (or M1) site, which creates reactivity and thermal expansion anisotropy issues. C. Y. Huang (Ph.D. Thesis, 1990) has computed and measured the thermal expansion anisotropies—the difference between axial thermal expansions in the ‘a’ and the ‘c’ directions of the unit cell—of such compositions and clearly shown the significantly higher anisotropy of the compositions with alkali metal ions (especially, Li+ and Na+) at the M1 or A site as compared to those with the larger alkaline earth ions such as Ca2+, Sr2+ and Ba2+ at these sites.
Notably also, the disclosed synthesis methods for the microbicide and antimicrobial inorganic phosphate compositions of the prior-art discussed above involve: (a) corrosive reagents (chlorides, sulfates, oxynitrates, oxychlorides, etc.) that produce environmentally-unfriendly effluents; and (b) tedious chemistries—sometimes with more than one iteration of digestion with carboxylic or dibasic acids such as oxalic and malic acid, pH-controlled reaction-precipitation, filtration, washing and controlled-drying.